Method of pickling titanium and compositions used therein



United States Patent 3,006,827 METHOD OF PICKLING TITANIUM AND COMPOSITIONS USED THEREIN Vincent J. Capuano, Hamden, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware No Drawing. Filed Jan. 6, 1959, Ser. No. 785,123

4 Claims. (Cl. 204141) This invention relates to a method for pickling titanium and titanium alloys and compositions used therein. More particularly this invention relates to a method for descaling and deoxidizing titanium and titanium alloys by the combined action of a pickling bath and electrolysis.

Oxides of titanium and oxide scale are formed on the surfaces of articles formed from titanium and titanium alloys when such articles are manufactured or fabricated by methods using heat treatment in the forming or tempering thereof. Proper finishing of such articles requires that the adherent scale and unsightly appearing discolorations be removed. Conventional methods of descaling are diflicult to carry out and often result in such damage to the article itself in way of etching, pitting, discoloration and decrease in the thickness of the articles that they were no longer fit for the purpose intended and have to be discarded.

Several conventional methods are known but the most common types are abrasion, contacting the oxidized surfaces With baths of molten salts or contacting such surfaces with acid pickling baths.

Abrasion cleaning is limited by the shape of the articles to be worked on. The articles having irregular, complex or tortuous surfaces are practically impossible to clean completely or evenly because of the inaccessability of portions of the surface. Molten salt treatment is not recommended since it is carried out at relatively high temperatures and usually result in discoloration of the article which requires a second treatment to remove the stain resulting from the first. The articles cleaned in this manner often are scratched, pitted and generally of a roughed appearance. The surface is usually of an uneven texture and the gauge thickness of articles so treated usually is worn down unevenly. Acid cleaning usually results in etched, pitted surfaces and from which complete removal of scale must be accomplished by mechanical means such as wire brushing.

An object of the present invention is to provide a method for pickling titanium and titanium alloys to remove oxide films and scale without severe treatment of the article.

Another object of the present invention is to provide a method and bath for pickling titanium and titanium alloys which does not substantially pit or etch the surface of the article treated but which results in the complete removal of oxide scale therefrom without removal of any appreciable amount of unoxidized metal.

A further object of the present invention is to provide a method of cleaning the surface of articles made of titanium and titanium alloys which combines chemical pickling with electrolytic treatment.

The present invention comprises using certain pickling bath compositions in conjunction with electrolytic treatment of the articles of titanium and titanium alloys. This combination permits the use of a pickling bath which is less severe in its attack on the metal surface but more selective in its attack on oxide scale when the article of titanium or titanium alloy is alternately used as the anode and then the cathode by periodically reversing the flow of current in a direct current electrolysis in which the pickling bath compositions form the electrolyte. The cathode is made from a titanium or a ferrous metal. Stainless steel, graphite, platinum and gold may also be used. The voltage of the direct current is in the range from 6 to 30 volts. The duration of this treatment varies in accordance with the amount of scale and stain present on the article.

The pickling bath compositions must be selected with the utmost care in order to be effective in removing oxide scale and brightening the metal surfaces without being so harsh as to pit, etch or materially decrease the surface of the articles to be treated therewith in conjunction with the application of direct current thereto. The following fonnulae have proven particularly effective in the present invention:

Water 88.5% by vol. Nitric acid (50-80%) 10.5% by vol. Hydrofluoric acid (48 60%) 1.0% by vol.

'Sulfamic acid 7-13% by wt. of liquids.

Cadmium sulfate 23.5% by Wt. of liquids.

Water 88.5% by vol. Nitric acid (5080%) 10.5% by vol. Hydrofluoric acid (48- 60%) 1.0% by vol.

Sulfamic acid 7-13% by wt. of liquids. Zinc sulfate 2-3.5 by Wt. of liquids.

III

Water 88.5% by vol. Nitric acid (5080%) 10.5% by vol. Hydrofluoric acid (48- 60%) 1.0% by vol. Sulfamic acid 713% by Wt. of liquids. Calcium sulfate 23.5% by wt. of liquids.

From the above examples, it is clear that the preferred nitric acid concentration of the pickling baths disclosed herein may vary from about 5.25 to 8.20 percent by volume, and that the preferred hydrofluoric acid concentration of the baths may vary from about 0.48 to 0.60 percent by volume.

Pickling baths of type I are preferred because of the overall uniformity of their descaling and brightening action. The baths of type II and type III have proven only slightly less efficient but quite satisfactory for the purposes of the present invention.

The present invention utilizes bath solutions made in accordance with the formulae set forth above in combination with galvanic action on the article to be descaled and cleaned. The combination produces the desired result using a milder etching solution operating more efficiently than heretofore. The acid attack on the metal surface is lessened apparently because of the inclusion of sulfamic acid and the group II metal sulfate buffer.

The baths active components being needed in less than conventional amounts makes unnecessary the storing of large amounts of hazardous material, usually in liquid form. The major component of the present invention, sulfamic acid (NH HSO is a dry material which can be stored in that condition until it is desired to use it in preparing the bath composition. The sulfate salts of group H metals are also used in the dry state until incorporated into the bath composition.

The present invention operates satisfactorily utilizing less than the amounts of active ingredients conventionally included in compositions designed to perform in a similar manner. This increase in effectiveness results in a substantial increase in the economies of a process which in commercial contemplation may use bath solutions in batches amounting to hundreds of gallons.

The invention in its broader aspects is not limited to the specific steps, methods, compositions and improvements shown and described herein, but departures may be made within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A method of removing oxide scale and stain from the surfaces of articles of titanium and titanium based alloys which comprises immersing the article in a picklingelectrolyte bath which consists essentially of an aqueous solution of about 5.25 to 8.40 percent by volume of nitric acid, about 0.48 to 0.60 percent by volume of hydrofluoric acid, about 7 to 13 percent by weight of sulfamic acid and about 2 to 3.5 percent by Weight of a sulfate salt of a metal of group II of the periodic table of elements, connecting the immersed article as an electrode in an electrical circuit, passing a direct current between said article anda second electrode and periodically reversing the flow of current.

2. A method of removing oxide scale and stain from the surfaces of articles of titanium and titanium based alloys which comprises immersing the article in a picklingelectrolyte bath which consists essentially of an aqueous solution of about 5.25 to 8.40 percent by volume of nitric acid, about 0.48 to 0.60 percent by volume of hydrofluoric acid, about 7 to 13 percent by weight of sulfamic acid and about 2 to 3.5 percent by weight of a sulfate salt of a metal of group II of the periodic table of elements, connecting the immersed article as an anode in an electrical circuit, passing a direct current having voltage of about from 6 to 30 volts between said article and a cathode, and periodically reversing the flow of current in said circuit.

3. A method as set forth in claim 1 wherein the anode is a metal selected from the group consisting of ferrous metals and titanium metals.

4. A method as set forth in claim 1 wherein the voltage of the current passed through the circuit is from 6 to 30 volts.

References Cited in the file of this patent UNITED STATES PATENTS 2,469,237 Mason et a1. May 3, 1949 2,538,317 Mason et a1 Jan. 16, 1951 2,550,388 Simon et a1 Apr. 24, 1951 2,711,389 Beach et al June 21, 1955 2,780,594 Dailey Feb. 5, 1957 2,798,843 Slomin et a1. July 9, 1957 2,801,813 Beuckman et a1. July 30, 1957 2,829,091 Missel Apr. 1, 1958 2,834,101 Broam May 13, 1958 OTHER REFERENCES Metallurgia, vol. 49, No. 295, May 1954, page 256. 

1. A METHOD OF REMOVING OXIDE SCALE AND STAIN FROM THE SURFACES OF ARTICLES OF TITANIUM AND TITANIUM BASED ALLOYS WHICH COMPRISES IMMERSING THE ARTICLE IN A PICKLING ELECTROLYTE BATH WHICH CONSISTS ESSENTIALLY OF AN AQUEOUS SOLUTION OF ABOUT 5.25 TO 8.40 PERCENT BY VOLUME OF NITRIC ACID, ABOUT 0.48 TO 0.60 PERCENT BY VOLUME OF HYDROFLUORIC ACID, ABOUT 7 TO 13 PERCENT BY WEIGHT OF SULFAMIC ACID AND ABOUT 2 TO 3.5 PERCENT BY WEIGHT OF A SULFATE SALT OF A METAL OF GROUP II OF THE PERIODIC TABLE OF ELEMENTS, CONNECTING THE IMMERSED ARTICLE AS AN ELECTRODE IN AN ELECTRICAL CIRCUIT, PASSING A DIRECT CURRENT BETWEEN SAID ARTICLE AND A SECOND ELECTRODE AND PERIODICALLY REVERSING THE FLOW OF CURRENT. 